Here are some of the experiences that I had while building one:
1) I used cedar for the body...I don't recommend anyone else try it! (Cedar was readily available to me, I have acres of it, that's the only reason I used it.) The problem at this point wasn't so much that cedar is a softer wood, it is the way cedar grows, and hence the way it dries. Unlike other woods, cedar is like a zillion small planks stacked upon each other from one side across the radius to the centre. When cedar dries, I discovered these 'planks' begin to split, leaving long splits from the top to the bottom of the body. I used some plumbers 'clamps' around the body to prevent it from splitting further.
2) I did not have a powerful enough drill to drill the holes for the arms and legs. I burned out two drills before I finally completed the holes. (Thank goodness for Canadian Tire, who take back tools with no questions asked!)
3) I did not examine the plan close enough (I had downloaded it from the Net). The plans were a mirror image of what they should have been, so when I started to install the first arm I realized 'Hey, something ain't right here...'. Back to square one, a new body. Fortunately the dead cedar I had taken down had enough length to get a second body from it.
4) I purchased some good maple table legs for the arms, and used a part of an old workbench apparatus for the leg. No problems with this stuff, it all was great.
5) After a year of banging on this thing, the softwood aspect of the cedar has lately started to become a problem. The back of the arm holes has gradually been wearing away with each smack of the arms, and now the arms have begun to fall out. I have used 'supports' at the back, kind of like the reinforcements you would use to support the pages of a three-ringed binder (but made of wood), but these break after a shot time, and I am forever replacing them.
6) Oh, ya...I have little motion in the body (some mooks are designed to slide side-to-side) because the only way I could mount it (get this, LOL) was to duct-tape it to a metal support beam in the basement. This actually works quite well, believe it or not! When I move around the body doing drills, it will move without falling over, it 'twists' slightly (around the pole), without toppling over!!
Those are some of my experiences building a mook jong.